Every so often I have this urge (maybe more of an itch) to spend hours and hours on the web trying to find information about old, obsolete computers of the past. I am intrigued by the XEROX Alto and Star ('70s-'82), the Apple Lisa ('83) and, of course, CRAYs ('75-ish). These were revolutionary machines indeed, they wrote golden pages in the history of computing. In the end of the 1980s, a new innovative product was ready to ship, created by a bunch of people coming from Apple: The NeXT platform.

Since MusicKit and SoundKit were mentioned in a comment, I should point out the MusicKit and SoundKit (now SndKit) were open sourced (well before this became a common tactic) around 1993 and have been running on all OpenStep variants (Rhapsody, Yellow Box, MacOSX Server 1.2, MacOS X DP1-4) and now runs on MacOS X, Linux and Windows. You can find more info at: http://www.musickit.org

Like GnuStep, we still have a way to go to complete the port, although what wasn't mentioned about GnuStep in the article is that is complete enough for most applications to port with very minimal effort, likewise for the MK/SK.